This invention relates to an improved load holding device for mobile power crane-excavators and the like. More particularly, the invention is directed to a device to improve the operation of check and relief valves inserted in the fluid line between a metering control valve and a function cylinder.
In power crane-excavators having a load carrying member such as a boom or dipper arm, a metering valve is often connected via fluid lines to the cylinder for the load carrying member to control the operation of the latter, such as in lowering and raising. For protection purposes, a one-way holding or check valve and a spring biased fluid relief valve have been inserted in parallel into the fluid line on the down side of the external force created by the weight of the load carrying member on the cylinder.
In such devices the metering valve is actuatable to connect pumped fluid to the desired cylinder side while the other cylinder side is opened to a fluid supply tank or drain. During lowering of the load carrying member, the fluid relief valve is overcome by the fluid pressure force. In addition, when the metering valve is in neutral position, the pump has been connected through the valve to the supply (or drain) tank to provide continuous circulation of fluid.
Heretofore, and upon shifting of the metering valve to neutral from an operating mode, certain undesirable phenomena have been observed. Pressure peaking and reverberations in both the said upside and downside lines have been noticed, which could cause line rupture, undesirable chatter-type opening of the check valve, as well as cyclic fatigue of the hydraulic system and structural load carrying members. In addition, it has been found that the fluid relief valve would not open under overload weight at the pressure it was set for, but rather required an additional amount of load pressure before opening. The design of these systems to operate properly under varying conditions of load and speed has therefor been rendered very difficult.
To the knowledge of the inventor, the cause of the aforementioned phenomena was previously unknown.
The present invention is based on the discovery by the present inventor of the said cause, and of a solution to the problems mentioned. In accordance with the invention, the cause was determined to be the fact that heretofore, and when the metering valve was in neutral, the said downside line was closed to the drain tank. Thus, when the valve was moved from an operating mode (wherein the down line was full of fluid) to neutral or load holding position, the fluid in the downside line would become suddenly trapped throughout the full length thereof, between the valve and cylinder. This was discovered to cause back pressures and the resultant pressure peaking and fluid reverberations. Furthermore, with no escape to drain available, the downside line was essentially permanently filled with pressurized fluid. The spring biased pressure relief valve therefor would not open at its normal set pressure, because any abnormal load weight would have to overcome not only the spring biasing force but also the fluid pressure in the blocked line before the valve would open.
After the discovery of the cause of the problem, the inventor has developed a simple yet unique solution to it. In accordance with the invention, the metering valve has been modified so that the downside line is opened to drain when the metering valve is in neutral. Thus, when the valve is shifted from operating mode to neutral, fluid in the down line downstream of the check and relief valves immediately drains to tank. Pressure peaking is substantially reduced and the relief valve will operate directly in accordance with its set spring biasing force.